FIELD ASSESSMENT OF NOVEL PV MODULE

21 downloads 0 Views 147KB Size Report
Dec 31, 2008 - end, four different novel PV module technologies (back-contact ... module given by a manufacturer) is the factor that has the most impact on the ...
FIELD ASSESSMENT OF NOVEL PV MODULE TECHNOLOGIES IN CANADA Yves Poissant CanmetENERGY, Natural Resources Canada 1615 Lionel-Boulet Blvd. Varennes, QC, J3X 1S6 Phone: +1 (450) 652-2471 Fax: +1 (450) 652-5177 Email: [email protected] ABSTRACT New PV module technologies have recently entered the Canadian market. As the energy output of these technologies is largely determined by the local climatic conditions where they are installed, it becomes important to obtain information on their actual field performance. To this end, four different novel PV module technologies (back-contact crystalline Silicon, laser-grooved buried junction crystalline Silicon, amorphous Silicon triple-junction, and heterojunction Silicon) were installed outdoor at the CanmetENERGY PV test facility at the Varennes Research Centre, near Montreal. Module power outputs at maximum power point, module temperatures and local climatic parameters (incident irradiance, global horizontal irradiance, direct and diffuse irradiance, ambient temperature) were measured every minute and averaged hourly from the period of January 1st to December 31st 2008. The aim of this paper is to report on the actual energy yield of these different module technologies. Furthermore, the energy yield within specific climatic conditions (low light levels, high diffuse content, high temperatures) are analyzed to provide insights on the behavior of these technologies and to verify whether some technologies perform better under these unique circumstances. These results are useful to provide guidance for PV module selection based on actual performance and to model the annual energy yield of these technologies. Overall, module tolerance (the uncertainty on the rated power of a module given by a manufacturer) is the factor that has the most impact on the yield of a PV module during operation. 1. INTRODUCTION The power of PV modules is rated under standard testing conditions (STC), defined as a module temperature of 25°C, an irradiance level of 1000 W/m2 perpendicular to the plane of the module and an AM1.5 spectral irradiance distribution. However, in the field, PV modules spend very little time operating under these conditions (typically less than 1% of their operational time!). As such, users nowadays tend to put more and more emphasis on actual energy delivered and the yield of a technology at a given location (kWh produced per rated kW capacity). Furthermore, the interest in actual field performance has led many novel technology manufacturers to make statements about the “superior performance” of their technology under a given climatic condition (e.g. low light levels, high diffuse content, warm summer months, etc.) [van Cleef, 2001]. Hence, it becomes important to test these novel technologies in the field and verify if, indeed, some technologies are more adapted to real operating conditions and produce higher energy outputs over a given period. This paper examines the performance of four novel PV technologies at a PV module test site near Montreal that have recently entered the Canadian market.

2. REAL OPERATING CONDTIONS Four different PV module technologies - laser grooved buried junction crystalline silicon (c-Si 1) from BP solar (BP585U), back contact crystalline silicon (c-Si 2) from Sunpower (SPR-90), triple junction amorphous silicon (a-Si) from Unisolar (US-32), and heterojunction silicon (h-Si) from Sanyo (HIP-190) were mounted on an open outdoor rack in Varennes [73° 22’ W, 45° 37’ N], Canada. The modules were oriented at latitude tilt (45 degrees), facing South. They were connected to a multitracer which operated the modules at their maximum power point, and which read PV output power and back-of-module temperature every 5 seconds, recording 1 minute averaged values from January 1st to December 31st 2008. IV curves were also traced for all modules at 5 minute intervals. In addition, various weather inputs were also read every 5 seconds, with 1 minute averages recorded. Measurement with precision pyranometers and pyrheliometers included global irradiance in the module plane, direct normal/beam irradiance, sky diffuse irradiance in a horizontal plane and global horizontal irradiance. Ambient temperature was also recorded.

Comparison of Global Incident Insolation Data 200 180 160 140 120 100 80 60 40 20 0

Meas. 1553 kWh/m2/yr PV Maps 1591 kWh/m2/yr RETScreen 1640 kWh/m2/yr CERES 1636 kWh/m2/yr

01 /2 00 02 8 /2 00 03 8 /2 00 04 8 /2 00 05 8 /2 00 06 8 /2 00 07 8 /2 00 08 8 /2 00 09 8 /2 00 10 8 /2 00 11 8 /2 00 12 8 /2 00 8

(kWh/m2)

Global Incident Monthly Insolation

Irradiance and module temperature are the main factors affecting the performance of PV modules. It is thus worthwhile to investigate these local climatic parameters in some depth. Figure 1 compares the monthly insolation totals measured by the inclined global pyranometer at the test site to three solar insolation sources: the Photovoltaic potential and solar resource maps of Canada1, RETScreen2 and the Canadian Renewable Energy Wind and Solar Resource (CERES)3 CD. Typically, the measured insolation monthly totals fell within one standard deviation of the normals, but the agreement was closer for the PV maps than the RETScreen or CERES sources. Although insolation and PV potential data exists for Varennes within NRCan’s PV maps, it is not the case for RETScreen or CERES. In those two cases, the closest weather station selected was the Saint-Hubert Airport, located about 15 km to the south of Varennes. This main explain why these two sources display higher yearly insolation totals (the yearly standard deviation is roughly 46 kWh/m2 for all three sources).

Figure 1. Comparison of the measured monthly global incident insolation data in the module plane to three solar insolation sources and their normals.

Aside from solar radiation, module temperature is the parameter that impacts the performance of a PV module the most and thus its energy production. Figure 2 presents the mean daily ambient temperature, the mean monthly module temperatures during operation for all four modules under study and their maximum monthly temperature. The annual mean ambient temperature for the test site was 8.2 °C; the mean yearly module temperatures during operation ranged between 20 to 22 °C; and maximum module temperatures ranged between 50-60 °C. Ambient, Mean and Maximum Module Temperatures 70,0

Temperature (°C)

60,0 50,0 40,0 30,0 20,0 10,0 0,0

Mean daily amb. T Mean a-Si T Mean h-Si T Mean c-Si 1 Mean c-Si 2 Max a-Si T Max h-Si T Max c-Si 1 T Max c-Si 2 T

01 /2 0 02 08 /2 0 03 08 /2 0 04 08 /2 0 05 08 /2 0 06 08 /2 0 07 08 /2 0 08 08 /2 0 09 08 /2 0 10 08 /2 0 11 08 /2 0 12 08 /2 00 An 8 nu al

-10,0

Figure 2. Mean daily ambient temperature, mean monthly temperatures during operation for all four modules under study and their maximum monthly temperature. 3. FIELD PERFORMANCE Figure 3 shows the monthly and annual DC energy production yields (energy per measured module nominal power – not rated) for the PV technologies under study from January 1st 2008 December 31st 2008 estimated from the average daily yields. Typically, all modules under study generated similar amounts of DC energy within a few percentage difference. The amorphous silicon module generated relatively more electricity in the summer (under high temperature) than in the winter (low temperature). Over the whole year, the difference in DC output between the most and the least productive module technology was 1.8%, which is not significant, considering the uncertainty on module rated powers (see section 4 for more details).

200 180 160 140 120 100 80 60 40 20 0

a-Si h-Si c-Si c-Si

total: 1570 Wh/W total: 1549 Wh/W 1 total: 1542 Wh/W 2 total: 1541 Wh/W

01 /2 0 02 0 8 /2 0 03 0 8 /2 0 04 0 8 /2 0 05 0 8 /2 0 06 0 8 /2 0 07 0 8 /2 0 08 0 8 /2 0 09 0 8 /2 0 10 0 8 /2 0 11 0 8 /2 0 12 0 8 /2 00 8

DC yields (Wh/W)

Module DC Power yields

Figure 3. Module DC monthly energy yields for year 2008 at the CanmetENERGY PV test facility in Varennes, Québec [73° 22’ W, 45° 37’ N]. The performance factor of a module is a parameter that is useful to characterize the field performance of a PV module. It is defined as the ratio of the module efficiency under field conditions to its efficiency under standard testing conditions (STC):

PF =

EOutput η Field SY = = η STC PSTC ⋅ φ φ

(1)

Figure 4 presents the monthly and annual performance factors for the PV module technologies under test. Under warm summer climate, the amorphous silicon technology performed relatively better; whereas under cold winter climate, the crystalline and heterojunction technologies showed a better performance. However, when one compares all four technologies on an annual basis, these monthly differences cancel each other and the performance spread amongst technologies is lower than 2%, which is again not significant enough to favor one technology over another for this test site as the module-to-module variability exceeds this spread. Another observation is that these novel technologies present performance factors close to 1, meaning that their performance under field conditions is equivalent to (and sometimes even better than) standard testing conditions, a clear improvement over certain past PV technologies [del Cueto, 2002].

Module Performance Factors

Performance factors

1,200 1,000 0,800 a-Si h-Si c-Si 1 c-Si 2

0,600 0,400 0,200

01

/2 0 0 2 08 /2 0 0 3 08 /2 0 0 4 08 /2 0 0 5 08 /2 0 0 6 08 /2 0 0 7 08 /2 0 0 8 08 /2 0 0 9 08 /2 0 1 0 08 /2 0 1 1 08 /2 0 1 2 08 /2 00 An 8 nu al

0,000

Figure 4. Module performance factor for year 2008 at the CanmetENERGY PV test facility in Varennes, Québec [73° 22’ W, 45° 37’ N]. Performance versus irradiance Figure 5 shows the distribution of annual irradiance in the module plane divided into irradiance bins. Although the PV module nominal rating is determined at an irradiance level of 1000 W/m2, 96% of the incident irradiation received annually is below that level. Keeping a good efficiency at all irradiance levels is thus an important characteristic for a module to ensure an adequate performance in the field. In addition, it is shown that although irradiance levels below 100 W/m2 represent the most populated irradiance bin (38.4% of the whole distribution), these low irradiance levels account for only 3.4% of the total solar energy received at the test site. In total, 39.4% of the annual solar energy is received at high irradiance level (>800 W/m2), 50.9% at medium level (201-800 W/m2) and 9.7% at low level (1 0 00 0

Distribution in module plane (%)

Annual Irradiance Distribution 45% 40% 35% 30% 25% 20% 15% 10% 5% 0%

Low

Medium

Irradiance (W/m2)

Figure 5. Distribution of annual irradiance in the module plane.

High

As shown on figure 6, all module technologies under investigation present similar trends: an efficiency that is fairly constant under medium irradiance levels, but decreases under low and high irradiance levels. As module performance factors are evaluated using the nominal efficiency or power determined at high irradiance level (1000 W/m2) which is lower than under medium irradiance, it explains why in some cases the module performance factors presented on figure 4 exceed 1. Module Efficiency vs Irradiance T= 25°C ±1°C

Efficiency (%)

25 20

a-Si

15

h-Si c-Si 1

10

c-Si 2

5 0 0

200

400

600

800

1000

1200

Irradiance (W/m2)

Figure 6. PV module efficiency versus irradiance for modules at 25°C. Temperature effects Figure 7 shows the distribution of annual PV module temperature during operation. All modules operated in a temperature range comprised between -20 °C to 57 °C. PV module nominal ratings are determined at a module temperature of 25 °C, but modules operated only 5% of the time in the 24-26 °C range. In 2008, modules spent 30% of their operational time above 26 °C, and they generated 55% of their output in this temperature range (high temperatures are associated with high irradiances); whereas they spent 65% of their operational time below 24 °C and generated 40% of their output in that temperature range.

Distribution (%)

PV Module Temperature Distribution during Operation 30% 25% 20%

a-Si h-Si c-Si 1

15% 10% 5% 0%

c-Si 2

< -10

-10 - 0

0 - 10

10 - 20 20 - 30 30 - 40 40 - 50 50 - 60

Temperature (deg C)

Figure 7. Annual PV module temperature distribution during operation for open rack mounted modules in Varennes.

Figure 8 presents module power as a function of module temperature using module power data obtained under an irradiance level of 950 – 1050 W/m2 that was corrected to 1000 W/m2. Except for the a-Si module, all module technologies present a linear decrease of power as their temperature increases. This thus explains why the a-Si module performs better in summer, whereas the h-Si and c-Si modules perform better under cooler temperature. Table 1 contains the Pmax temperature coefficient obtained from figure 8 for the different technologies (see [Poissant, 2003] and [King, 1997] for more details on temperature coefficients). Module Power vs Module Temperature (Data corrected to 1000 W/m2) 250 a-Si

200 Power (W)

h-Si c-Si 1

150

c-Si 2 100 50 0 0

10

20

30

40

50

60

Module Temperature (deg C)

Figure 8. PV module power versus module temperature. Table 1 Module parameters obtained from Figure 8 Module Parameter Pmax T coeff. (%/°C) Pmax actual (W) Pmax actual – Pmax rated (W)

a-Si

h-Si

c-Si 1

c-Si 2

0.270

-0.296

-0.439

-0.473

24.5

177.6

75.3

83.3

-7.5 W (-23%)

-12.4 W (-6.5%)

-9.7 W (-11.4%)

-6.7 W (-7.4%)

Rated power versus actual power The rated or nominal power of a PV module is determined under standard testing conditions (module temperature of 25°C, irradiance level of 1000 W/m2 perpendicular to the plane of the module and an AM1.5 spectral irradiance distribution). As the spectral distribution of the data represented on figure 8 can be approximated to AM1.5 as the dataset consists only of high irradiance data, it is possible to determine the nominal power of the modules from this figure. Table 1 lists the module powers at maximum power point and compares these values to the rated values from the module manufacturers. All modules under study underperform their manufacturer rating by 6.5% to as high as 23%. Furthermore, as shown on figure 9, the case of amorphous

silicon is more complex as its maximum power point varies throughout the year. The average Pmax for the a-Si module was 25.2 W, with a low of 24.4 W in April and a maximum of 26.2 W in September. These variations represent about 4% of the average Pmax value and could be linked to the metastability of amorphous silicon (see [King, 2002] & [del Cueto, 1999] for more details).

a-Si Pmax 30,0 Pmax (W)

25,0 20,0 15,0 10,0 5,0

-0 8

05

-0 8

04

-0 8

03

-0 8

-0 8

02

-0 7

01

-0 7

12

11

-0 7

10

-0 7

-0 7

09

08

07

-0 7

0,0

Month

Figure 9. Variation of a-Si module STC Pmax throughout the year. 4. DISCUSSION What can we conclude about the performance of these novel PV module technologies? Is there a technology that is better adapted to southern Canadian climatic conditions? This study showed an equivalent performance for these technologies under different irradiance levels and confirmed that the multijunction amorphous silicon and heterojunction silicon technologies are not as affected by temperature than other novel crystalline silicon technologies. While these characteristics may lead to different power productions on single days (e.g. sunny day with cold vs warm ambient temperature) or even months (winter months vs summer months), these differences tend to cancel out over the full annual period. These field observations apply to mild Canadian climatic conditions and may be different in northern parts of Canada (above 60º latitude) where crystalline silicon may benefit from colder ambient temperatures. Or, one may be interested in optimizing the PV production for peak shaving during hot summer peaks where amorphous silicon would perform better than other technologies. In more general cases, one should put manufacturer claims into perspective. Even if a technology has a slight advantage under low irradiance level or diffuse irradiance (something that was not observed in this study), the annual low light irradiation represents less then 10% of the total insolation received during a year. Good performance under medium and high irradiance is far more important on a yearly basis. In light of analyses, the ability (or inability) of manufacturers to produce modules meeting their nominal power (i.e. respecting their rated power) is by far the factor that will impact the most the energy yield at any given site. In this study, all performance parameters were normalized to the actual observed power in the field, thus the results presented here are not affected by this effect

and the comparisons between technologies remain valid. However, consumers only have access to the rated module power. All modules investigated here underperformed their rated power by 6.5% to as high as 23%. Thus, if consumers had purchased these modules, installed them in the field and monitored their electricity production, their conclusions on performance ratios and specific yields would have been quite different. Table 2 compares the parameters obtained from field measurements to those obtained using rated values given by manufacturers. Although it is risky to draw conclusions on the absolute performance value of a given technology based on only one sampled module, this exercise demonstrates the importance of selecting modules that have small tolerance values (i.e. low Pmax uncertainty) and the caution one should exercise when relying on manufacturer parameters for estimating energy production. Table 2 Module parameters obtained from rated values versus real values Module Parameter Pmax STC

(W) Tolerance (%) Efficiency STC (%) Temp.coeff. (%/°C) Annual specific yield (Wh/W) Annual perfor. factor

Rated value (Actual value) Rated value (Actual value) Ratings based value (Actual value) Rated value (Actual value) Ratings based value (Actual value) Ratings based value (Actual value)

a-Si 32 W (25.2 W) ±10% (-23%) 6.1% (4.7%)

h-Si 190 W (177.6 W) 0 to +10% (-6.5%) 16.5% (15.5%)

c-Si 1 85 W (75.3 W) ±6% (-11.4%) 13.1% (11.6%)

c-Si 2 90 W (83.3 W) ±5% (-7.4%) 16.6% (15.4%)

-0.21%/°C -0.30%/°C -0.50%/°C -0.38%/°C (+0.27%/°C) (-0.296%/°C) (-0.439%/°C) (-0.473%/°C) 1237 Wh/W 1448 Wh/W 1366 Wh/W 1427 Wh/W (1570 Wh/W) (1549 Wh/W) (1542 Wh/W) (1541 Wh/W) 0.799 (1.01)

0.931 (0.996)

0.879 (0.992)

0.918 (0.991)

5. CONCLUSION The performance of four novel technologies has been evaluated in the field from January 1st 2008 to December 31st 2008 at a test location near Montreal. No significant performance difference amongst technologies were observed under low or diffuse light. This study however confirmed that the multijunction amorphous silicon and heterojunction silicon technologies are not as affected by temperature than other novel crystalline silicon technologies. As a result, small seasonal performance differences are observed between different technologies when one normalizes performance parameters using nominal module powers measured in the field (not the rated power given by manufacturers). On an annual basis however, these differences tend to cancel out and performance differences become insignificant. Thus, for southern canadian locations (conclusions may be different under arctic climate), one can conclude that these technologies will produce similar annual DC electricity yields. Compared to irradiance and module temperature, which are usually stated as being the main factors affecting the performance of PV modules, module tolerance (the uncertainty on the rated power of a module given by a

manufacturer) is the factor that has the most impact on the yield of a PV module during operation. 6. ACKNOWLEDGEMENTS Financial support for this research project was provided by Natural Resources Canada through the Program on Energy Research and Development. 7. NOMENCLATURE η: PV module efficiency Φ: insolation (kWh/m2) / 1 kWp·m-2 PSTC : module nominal power (in kWp) EOutput : electricity output (kWh) SY: specific yield (kWh/kWp) 8. REFERENCES 1

Photovoltaic potential and solar resource maps of Canada: https://glfc.cfsnet.nfis.org/mapserver/pv/index.php 2

RETScreen: http://www.retscreen.net/

3

The Canadian Renewable Energy Wind and Solar Resource (CERES) CD contains monthly values of solar radiation on the horizontal and on surfaces of various orientations for over 150 Canadian locations. The CD-ROM is available from Environment Canada: http://www.climate.weatheroffice.ec.gc.ca/

del Cueto J.A and von Roedern, B. (1999) “Temperature-induced Changes in the Performance of Amorphous Silicon Multi-junction Modules in Controlled Light-soaking” Prog. Photovolt: Rs. Appl. 7, 101-112. del Cueto J.A. (2002) “Comparison of Energy Production and Performance from Flat-Plate PV Module Technologies deployed at fixed tilt” Proc. 29th IEEE PVSC, New Orleans, USA. King D.L., Kratochvil J.A. & Boyson W.E. (1997) “Temperature Coefficients for PV Modules and Arrays: Measurement Methods, Difficulties, and Results” Proc. 26th IEEE-PVSC, Anaheim, USA. King D.L., Kratochvil J.A. & Boyson W.E. (2002) “Stabilization and Performance Characteristics of Commercial Amorphous-Silicon PV Modules” Proc. 28th IEEE-PVSC, New Orleans, USA. Poissant Y. et al. (2003) “A Comparison of PV Module Energy Performance under Various Environmental Conditions” Proc. SESCI 2003 conference, Kingston, Canada. Aug. 18th – 20th 2003. van Cleef M., Lippens P. & Call J. (2001) “Superior Energy Yields of UNI-SOLAR® Triple Junction Thin Film Silicon Solar Cells compared to Crystalline Silicon Solar Cells under Real Outdour Conditions in Western Europe” Proc. 17th EPVSEC, Munich, Germany.